The use of mobile phones and wireless broadband has increased rapidly during the last decade, and it is expected to grow even faster coming years. To meet these demands and stay competitive, the operators are forced to exploit every possibility to increase the capacity in their communication systems. One way to do this is to deploy more base stations and use smaller cells, which enables a higher frequency reuse and an increased capacity of the system. Unfortunately, with many small cells and dense deployment of base stations, the co-channel interference between different cells is increased. The large number of base stations also gives rise to a high energy consumption for the system as a whole which is not only expensive, but also has a negative effect on the environment.
Due to the above, manufactures of communication systems and equipment are faced with a number of difficulties that need to be overcome, one of which is the inter-cell interference caused by the dense deployment. One known way of decreasing inter-cell interference in a communication system is to use so called beam tracking. Beam tracking means that with help of an antenna array at a base station, the base station forms a narrow beam that tracks each individual user terminal. The direction of the tracking beam must be able to change quickly in order to track a user terminal; therefore the excitation weights of the antenna array that steer the beam are usually controlled by the base band. Since the beam towards the user terminal becomes narrower, less power will be directed towards other user terminals and therefore the interference will be reduced.
Another obstacle to master is to increase the coverage of the base stations of a communication system without necessarily increasing the transmit power of the base stations. One way to increase the capacity or coverage of a communication network is to use reconfigurable antennas at the base stations. A reconfigurable antenna is an antenna that can change its properties, by for example changing beam width, beam direction etc. With help of reconfigurable antennas, the base station can for example steer its beam towards hot spot areas, and therefore increase the Signal to Interference Noise Ratio (SINR) at the user terminals. Reconfigurable antennas usually do not need to change the beam as quickly as for beam tracking. The reconfigurable antenna usually just follows the traffic distribution of the user terminals and therefore may be needs to change the beam on minute or hour basis. Phase shifters and power amplifiers are usually used to shape different beams.
A further problem with existing communication networks is that the base stations often use unnecessary large transmit power when communicating with user terminals, without necessarily achieving better performance. The large transmit power used by the base station will contribute to a large signal power received by the user terminals, which of course is good for the performance. However, if the user terminals in the mobile communication system are interference limited instead of noise limited, the large transmit power of the base stations will also contribute to higher interference to other user terminals. This means that the high transmit power used by the base stations does not necessarily increase the SINR.
A known solution in prior art [1] presents a method of reducing the downlink transmission power level in a communication system. A user terminal measures and reports its noise and interference to a serving base station e.g. Node B. Based on the reported measurements, statistics are made and a decision is made whether to decrease or increase the transmitting power in the serving base station or not. This will change the coverage areas of the surrounding Node Bs, and potentially lead to handovers and load balancing. It is possible to include the surrounding Node Bs in the process of determining optimal power settings to avoid the above mentioned problems. However, in the case of noise limited adjacent cells this is not possible. The prior art then presents a load indicator. Unfortunately, this may lead to altering a planned power reduction or even abandoning the power reduction.
Consequently, there is a need for methods and arrangement for reducing transmission power, and in addition reducing the inter-cell interference, and also for improving overall coverage in a communication system.